Semiconductor light emitting device with light extraction surface

ABSTRACT

A semiconductor light emitting device which includes at least one concave on a light extraction surface opposite to a surface on which a semiconductor stack comprising a light emitting layer between a n-type semiconductor layer and a p-type semiconductor layer is mounted. The concave has not less than two slopes each having a different slope angle in a direction that a diameter of the concave becomes narrower toward a bottom of the concave from an opening of the concave and a slope having a gentle slope angle is provided with irregularities and a slope having a steep slope angle is a flat surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the foreign priority benefit under Title 35,United States Code, §119(a)-(d) of Japanese Patent Application Number2008-241798, filed on Sep. 19, 2008, the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light emitting device,and more particularly relates to a semiconductor light emitting devicewhich is provided with concaves/convexes on a surface for a lightextraction (hereinafter, referred to as light extraction surface) forincreasing light extraction efficiency.

2. Description of the Related Art

Conventionally, a compound semiconductor, for example, a gallium nitride(GaN) has been used as a common material for a semiconductor lightemitting device such as an LED. In the semiconductor light emittingdevice described above which uses a GaN-based compound semiconductor, aplurality of concaves/convexes may be formed on a semiconductor layersurface (see paragraph [0050] and FIG. 1 in JP 2000-196152, andparagraphs [0054], [0088], FIG. 1 and FIG. 20 in JP 2007-88277) or asubstrate (see paragraph [0042] and FIG. 2 in JP 2003-69075, paragraph[0045] and FIG. 3(b) in JP 2007-67209, and paragraphs [0054], [0088],FIG. 1 and FIG. 20 in JP 2007-88277) for increasing light extractionefficiency of a semiconductor layer which is stacked including aGaN-based light emitting layer in most cases.

In the light emitting device disclosed in JP 2000-196152, manyhemispherical convexes are formed spaced each other on a lightextraction surface of a GaN-based semiconductor layer which is grown ona sapphire substrate. This is because if the light extraction surface isa flat surface, an obliquely incident light on the flat surface istotally reflected. In the light emitting device, the convex enables theobliquely incident light on the light extraction surface (convex) topass outside, depending on an angle between a surface of the convex andthe incident light.

In the light emitting device disclosed in JP 2003-69075, a substrate iscomposed of a GaN-based compound semiconductor. In the GaN-basedcompound semiconductor substrate, a pit (hole, concave) is formed on asurface opposite to a surface on which a device made of, for example,GaN-based semiconductor layer is formed. In the pit, for example, aplurality of planes appear in a step-like pattern and a slope is formedbetween the planes.

In the light emitting device disclosed in JP 2007-67209, a undoped GaNsubstrate which has a dislocation density not more than 10⁶/cm² is used.In the undoped GaN substrate, macro concaves/convexes (step: 3 μm) areformed by grinding using a grinder on a surface opposite to a surface onwhich a device made of, for example, GaN-based semiconductor layer isformed, and the ground surface is further chemically treated (dryetching) to form circular cones in high density as microconcaves/convexes (step: not more than 1 μm). In the JP 2007-67209, thefinal purpose is to realize a high light emitting efficiency, whiletargeting a reduction of a driving voltage of the light emitting deviceas a firsthand purpose by disposing the micro convexes in high density.Hence, the light emitting device is provided with the macro convexes ina pre-treatment for disposing the micro convexes in high densityfinally.

The light emitting device disclosed in JP 2007-88277 is a face-down typedevice (flip chip). Two types of convexes are formed using a patterneither on a surface opposite to a surface on which a GaN-basedsemiconductor layer is formed on a sapphire substrate or on a surface ofn-type semiconductor layer after removal of the sapphire substratesubsequent to a formation of each electrode. One is a first convex (1μm) which is formed in a long-period and relatively high. The other is asecond convex (0.3 μm) which is formed in a short-period and relativelylow. In the technology disclosed in JP 2007-88277, an interval betweenconvexes is set such that, for example, the long-period is not less than2.3 μm and the short-period is not less than 0.46 μm, based on theexperimental data to increase the light extraction efficiency.

However, when concaves/convexes are disposed for increasing the lightextraction efficiency, there are various problems depending on astructure of the concaves/convexes. For example, in the light emittingdevice disclosed in JP 2000-196152, since the convex formed on asemiconductor layer surface is formed in a hemispherical shape, thelight extraction efficiency of a light emitted outside from the convexin the right upward direction becomes low, thereby resulting in poorlight distribution.

If the structure is a concave, it is expected that light extractionefficiency may be increased by making the concave deeper. For example,when an opening area (surface side portion) of the concave is fixed andthe concave has a taper shape in a depth direction from the opening, andif a surface (side surface) of the concave in the depth direction issteeply inclined, a deeper concave may be formed. However, when anelectrode (n-electrode or p-electrode) is formed on a semiconductorlayer surface where a plurality of concaves/convexes are disposed as theconventional technology, if a deep concave is formed, a currentdiffusion from the electrode is likely to become poor due to the deepconcave even if the electrode is not disposed right above the concave.As a result, there is a tendency that a current flowing downward fromthe electrode becomes dominant in the total current on the horizontalplane. In addition, to uniformly increase the current diffusion in thehorizontal direction from the electrode, it is necessary to increase afilm thickness of the semiconductor layer, where the concave isdisposed, right beneath the electrode in comparison with a semiconductorlayer where the concave is not disposed.

When the electrode is further stacked as described above, it isnecessary to form a depth of the concave relatively shallow by making aninclination of the side surface of the concave gentle. It may bepossible to form an opening portion of the concave larger in accordancewith a shortened depth of the concave by making the inclination of theside surface of the concave gentle. In this case, however, theconcaves/convexes become relatively small, and as a result, the lightdistribution becomes poor due to approaching to a flat surface.

The pit as a concave of the light emitting device disclosed in JP2003-69075, on which an electrode is stacked, is disposed forsuppressing an interference to be generated by multiple reflection oflight inside the semiconductor device. Hence, in JP 2003-69075, there isno description on what structure of the pit improves the lightextraction efficiency.

In the light emitting device disclosed in JP 2007-67209, a convex isformed on a backside of an undoped GaN substrate for achieving areduction of a driving voltage, which is a firsthand purpose. Namely,the convex is not disposed on a light extraction surface of a GaN-basedsemiconductor layer surface. Therefore, there is no direct relationbetween a convex structure and light extraction efficiency of a surfaceon the light extraction side of a semiconductor layer including a lightemitting layer.

With respect to the light emitting device disclosed in JP 2007-88277, itis required that positions of a first convex and a second convex areaccurately arranged based on a unique theory and experimental data forimproving the light extraction efficiency. In addition, a light emittingdevice other than a facedown type (flip chip) can not be applied to thelight emitting device disclosed in JP 2007-88277.

The present invention has been developed considering the foregoingproblems, and it is an object of the present invention to provide asemiconductor light emitting device which has high light extractionefficiency of a surface on a light extraction side of a semiconductorlayer including a light emitting layer and a good light distribution.

SUMMARY OF THE INVENTION

To solve the foregoing problems, according to a first aspect of thepresent invention, there is provided a semiconductor light emittingdevice which has at least one concave on a light extraction surfaceopposite to a surface on which a semiconductor stack including a lightemitting layer between a n-type semiconductor layer and a p-typesemiconductor layer is mounted. The concave has not less than two slopeseach having a different slope angle in a direction that a diameter ofthe concave becomes narrower toward a bottom of the concave from anopening of the concave. A slope having a gentle slope angle is providedwith irregularities and a slope having a steep slope angle is a flatsurface.

In the configuration described above, since the semiconductor lightemitting device has a concave on the light extraction surface that is asurface of a semiconductor layer including the light emitting layer, ifan electrode (n-electrode or p-electrode) is stacked on thesemiconductor layer where the concave is disposed, a current diffusionfrom the electrode is good (improved) in comparison with a case where aconvex is disposed. The reason is as follows. When a plurality ofconvexes are disposed, two adjacent convexes are connected by only aportion of the semiconductor layer below the bottom of the convexes. Onthe other hand, when a plurality of concaves are disposed, two adjacentconcaves are connected by a portion of the semiconductor layer above thebottom of the concave. Therefore, a current actually flows in thelateral direction on a surface of the semiconductor layer above theconcave on which the electrode is disposed. As a result, a uniform lightemission can be obtained.

In addition, in the configuration, since the semiconductor lightemitting device has at least two slopes each having a different slopeangle in the direction where a diameter of the concave becomes narrowertoward the bottom, if a light emitted from the light emitting layerenters a steep slope, the light is output in the upper direction of theconcave by refraction, or the light is output in the upper direction ofthe concave by reflection at another steep slope facing the steep slopeafter the refraction. In addition, since the semiconductor lightemitting device has at least two slopes, a number of reflection can bereduced in comparison with a conventional structure where a number ofslope is one and multiple reflection is caused, and as a result, thelight is efficiently output in the upper direction.

In addition, in the configuration, since the slope having a gentle slopeangle is provided with the irregularities and the slope having a steepslope angle is a flat surface in the concave of the semiconductor lightemitting device, if a light emitted from the light emitting layer entersand transmits the slope having a gentle slope angle, the transmittedlight is scattered. Therefore, even if a light emitted form the lightemitting layer obliquely enters the slope having a gentle slope angle, acomponent of the light propagating in the right upper direction in thetotal light emitted outside increases. Therefore, the light distributionbecomes good, as well as the light extraction efficiency is improved.Furthermore, since a light emitted from the light emitting layer isefficiently reflected when the light enters the slope having a steepslope angle, a light to be emitted outside increases, resulting inimprovement of light extraction efficiency.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that the slope of the concave has agentler slope angle as the slope becomes closer to the opening of theconcave.

In the configuration, since the slope of the concave of thesemiconductor light emitting device is formed to be gentler as the slopebecomes closer to the opening of the concave, a probability that a lightonce reflected at a slope is continuously reflected many times isreduced in comparison with a reverse case where the slope is formed tobe gentler as the slope becomes closer to the bottom of the concave. Inother words, since the light emitting device has a slope having agentler slope angle as the slope becomes closer to the opening of theconcave, a light emitted outside is likely to be extracted in the upperdirection at a steep angle in comparison with a case where the slope isformed to be gentler as the slope becomes closer to the bottom of theconcave. Furthermore, in the semiconductor light emitting device, sincethe slope angles of the respective slopes of the concave have therelationship described above, the current is easily diffused, and as aresult, a uniform light emission can be achieved (obtained) incomparison with a case where the concave is merely disposed.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that the concave has a bottomsurface at a bottom and the bottom surface is provided with theirregularities.

In the configuration, since the concave of the semiconductor lightemitting device has a bottom surface, an optical path length of a lightwhich is emitted from the light emitting layer, enters a slope near thebottom of the concave from outside the concave, passes through insidethe concave and re-enters a slope facing the foregoing slope becomeslong in comparison with a case where the concave has no bottom surface.If the concave has no bottom surface, a light which is emitted from thesemiconductor layer and enters a slope near the bottom of the concavefrom outside the concave passes through the concave in a short time andre-enters an adjacent semiconductor layer from a slope facing theforegoing slope. On the other hand, in the semiconductor light emittingdevice that the concave has a bottom surface, the light receives moreeffect of refraction inside the concave by the prolonged optical pathlength due to the existence of the bottom surface, and a reflected lightat a slope facing the foregoing slope may enter another different slopeclose to the opening of the concave. Namely, re-entering of a lightemitted from a semiconductor layer into the semiconductor layer can bereduced.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that a plurality of the concaves aredisposed on the light extraction surface, adjacent openings of theplurality of the concaves are separated each other, and a separatingarea which separates the adjacent openings is provided with theirregularities.

In the configuration, since a plurality of concaves of the semiconductorlight emitting device are disposed so that the adjacent openings areseparated each other, a current diffusion on the light extractionsurface becomes good when an electrode is disposed on the lightextraction surface. In addition, the light distribution becomes good incomparison with a case where the separating area between the openings ofthe plurality of concaves is a flat surface.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that an electrode is disposed on thelight extraction surface.

In the configuration, a current diffusion from the electrode in thesemiconductor light emitting device becomes good in comparison with acase where a convex is disposed. Here, it is preferable that theelectrode is disposed in an area other than the concave on the lightextraction surface. By the arrangement of the electrode described above,the light extraction efficiency is increased since the light is notprevented from extracting by the electrode. In addition, since aplurality of electrodes are dispersively arranged, a current in thesemiconductor layer is easily and uniformly diffused.

In addition, in the semiconductor light emitting device according to thepresent invention, a shape of the opening of the concave may be formedin a circle in plan view of the light extraction surface.

In the configuration, the slope of the concave can be easily tapered.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that a shape of the opening of theconcave is a polygon in plan view of the light extraction surface.

In the configuration, openings of adjacent concaves in plan view can bearranged in contact with each other by a point or line in high density.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that a shape of the opening of theconcave is a hexagon in plan view of the light extraction surface.

In the configuration, with respect to an arrangement of a plurality ofconcaves, respective centers of the plurality of concaves 90 are in linein each row and in line every other column, and the plurality ofconcaves whose openings are in contact with each other by a tangent lineor a point can be close-packed. Namely, many concaves can be effectivelyand integrally arranged in a small surface area.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that a depth of the concave has alength not less than half of a thickness between the light extractionsurface and the light emitting layer and less than the thickness betweenthe light extraction surface and the light emitting layer.

In the configuration, since the slope of the concave is close to thelight emitting layer, the light can be efficiently extracted outsidefrom the semiconductor layer.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that the slope of the concave iscovered with a passivation film.

In the configuration, the concave which has an important role for thelight extraction can be protected. Here, it is preferable that thepassivation film is formed by a material having a refractive indexbetween those of the semiconductor layer on the slope side and anoutside material on the opposite side of the slope so that the light iseasily extracted. For example, if the outside is the atmosphere, it ispreferable that the passivation film is formed by SiO₂ or Al₂O₃.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that the concave is disposed in then-type semiconductor layer.

In the configuration, since a film thickness of the n-type semiconductorlayer is thicker than that of the p-type semiconductor layer, theconcave can be formed deeper. In addition, a relatively deep concave canbe formed in the total thickness of the semiconductor light emittinglayer. As a result, a light can be efficiently extracted outside fromthe semiconductor layer. In addition, in the configuration, since then-electrode stacked on the n-type semiconductor layer is small in sizein comparison with the p-electrode, when a plurality of n-electrodes aredispersively arranged, the n-electrodes can be easily arranged so that acurrent uniformly diffuses in the semiconductor layer.

In addition, in the semiconductor light emitting device according to thepresent invention, it is preferable that the concave is disposed at aposition facing a p-electrode which is disposed on the surface on whichthe semiconductor stack is mounted.

In the configuration, a light emitted from the light emitting layertoward a surface opposite to the light extraction surface is reflectedat the p-electrode, and the reflection light is easily extracted outsidethe semiconductor layer from the concave. In addition, in theconfiguration, a plurality of n-electrodes can be dispersively arrangedin consideration of a current diffusion to be determined depending onthe arrangement of the p-electrode. For example, when a plurality ofn-electrodes are disposed on an area other than the concave, thep-electrode and the n-electrode are arranged so that the p-electrodedoes not face the n-electrode in the depth direction of thesemiconductor stack. Therefore, a current easily diffuses, especially,in the semiconductor layer. On the other hand, when the p-electrodefaces the n-electrode in the depth direction of the semiconductor stack,a current is likely to flow along a path connecting the p-electrode andthe n-electrode, and the current does not uniformly flow as seen in thehorizontal direction, resulting in generation of uneven currentdistribution.

According to the present invention, a semiconductor light emittingdevice is provided, which can easily extract a light outside from asurface on a light extraction side that is a surface of a semiconductorlayer including a light emitting layer, and has a good lightdistribution. In addition, according to the present invention, if anelectrode is stacked on a semiconductor layer on which the concave isdisposed, a current easily and uniformly diffuses in the semiconductorlayer. Since the current easily and uniformly diffuses in thesemiconductor layer, it is unnecessary that a semiconductor layer, wherethe concave just beneath the stacked electrode is disposed, has anexcessive thickness. In addition, since the current easily and uniformlydiffuses in the semiconductor layer, a depth of the concave can be madeshallower in comparison with the conventional one. In this case, thelight extraction efficiency can be further improved, accordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view schematically showing a structure of asemiconductor light emitting device according to an embodiment of thepresent invention;

FIG. 2 is a cross sectional view schematically showing an enlargedconcave shown in FIG. 1;

FIG. 3 is a view schematically showing a plane, X-X cross section andY-Y cross section of a concave;

FIG. 4 is a perspective view schematically showing a partial crosssection of a concave shown in FIG. 3;

FIG. 5A to FIG. 5D are cross sectional views schematically showing acomparison between a concave of a semiconductor light emitting deviceaccording to the present embodiment and concaves other than the presentembodiment, where FIG. 5A shows the concave according to the presentembodiment and FIG. 5B to FIG. 5D show comparative examples;

FIG. 6 is a graph showing a relationship between a roughness of a lightextraction surface and a light directionality;

FIG. 7 is a graph showing an example of a light directionality of asemiconductor light emitting device according to the embodiment;

FIG. 8A to FIG. 8F are cross sectional views schematically showingfabrication processes of the semiconductor light emitting device shownin FIG. 1 (first);

FIG. 9A to FIG. 9E are cross sectional views schematically showingfabrication processes of the semiconductor light emitting device shownin FIG. 1 (second);

FIG. 10 is a flowchart showing formation processes of the concave shownin FIG. 9C;

FIG. 11 is a view schematically showing a cross section in a depthdirection of a first modified example of a concave structure;

FIG. 12 is a view schematically showing a cross section in a depthdirection of a second modified example of a concave structure;

FIG. 13 is a view schematically showing a plane, X-X cross section andY-Y cross section of a first modified example of an opening of aconcave;

FIG. 14 is a perspective view schematically showing a partial crosssection of the concave shown in FIG. 13;

FIG. 15 is a view schematically showing a plane, X-X cross section andY-Y cross section of a second modified example of an opening of aconcave;

FIG. 16 is a perspective view schematically showing a partial crosssection of the concave shown in FIG. 15;

FIG. 17 is a view schematically showing a plane, X-X cross section andY-Y cross section of a third modified example of an opening of aconcave; and

FIG. 18 is a perspective view schematically showing a partial crosssection of the concave shown in FIG. 17.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, a best mode for embodying a semiconductor light emittingdevice according to the present invention (referred to as embodiment)will be explained by referring to drawings. It is noted that thicknessesand lengths of, for example, constituents shown in the drawings areenlarged for clearly explaining the arrangements.

[Structure of Light Emitting Device]

A light emitting device according to an embodiment of the presentinvention relates to the light emitting device where at least oneconcave is disposed on a light extraction surface opposite to a surfaceon which a substrate of a semiconductor stack having a light emittinglayer between a n-type semiconductor layer and a p-type semiconductorlayer is mounted. First, a structure of the semiconductor light emittingdevice will be explained by referring to FIG. 1 to FIG. 4. FIG. 1 is across sectional view schematically showing a structure of asemiconductor light emitting device according to an embodiment of thepresent invention, and FIG. 2 is a cross sectional view schematicallyshowing an enlarged concave shown in FIG. 1. In addition, FIG. 3 is aview schematically showing a plane, X-X cross section and Y-Y crosssection of a concave, and FIG. 4 is a perspective view schematicallyshowing a partial cross section of the concave shown in FIG. 3.

As shown in FIG. 1, a semiconductor light emitting device 1 according tothe embodiment mainly consists of a substrate 10, a metallization layer20, a p-electrode 30, a passivation film 40, a semiconductor stack 50,an n-electrode 60, a passivation film 70 and a backside metallizationlayer 80.

(Substrate)

The Substrate 10 is Made of Silicon (Si). Meanwhile, Other than Si, forexample, a semiconductor substrate made of a semiconductor such as Ge,SiC, GaN, GaAs, GaP, InP, ZnSe, ZnS and ZnO, or a single metalsubstrate, or a metal substrate made of metal complex which is composedof not less than two metals which are mutually immiscible or have asmall solid solubility limit may be used. As a single metal substrate,specifically, a Cu substrate can be used. In addition, as a metalsubstrate, specifically, a substrate which is composed of at least onemetal selected from a highly-conductive metal such as Ag, Cu, Au and Ptand at least one metal selected from a high hardness metal such as W,Mo, Cr and Ni may be used. When a semiconductor material substrate 10 isused, a device, for example, a zener diode may be added to the substrate10. Further, as a metal substrate, a complex of Cu—W or Cu—Mo may bepreferably used.

(Metallization Layer)

A metallization layer 20 is a eutectic alloy for bonding two substratesin a fabrication process of the semiconductor light emitting device 1.Specifically, a metallization layer 21 on the epitaxial side shown inFIG. 8D and a metallization layer 22 on the substrate side shown in FIG.8E are bonded to form the metallization layer 20. The metallizationlayer 21 on the epitaxial side is formed by stacking, for example,Ti/Pt/Au/Sn/Au in this order from the bottom in FIG. 8D. In addition,the metallization layer 22 on the substrate side is formed by stacking,for example, Au/Pt/TiSi₂, or TiSi₂/Pt/Pd in this order from the top inFIG. 8E.

Returning to FIG. 1, explanation for the structure of the semiconductorlight emitting device 1 will be continued.

(p-Electrode)

A p-electrode 30 is formed on a mounting surface of a semiconductorstack 50 on the side of substrate 10 and at positions facing theconcaves 90 on the uppermost surface of the semiconductor stack 50 inthe depth direction.

Specifically, the p-electrode 30 consists of at least two layers, ap-electrode first layer (not shown) on the side of semiconductor stack50 and a p-electrode second layer (not shown) on the lower side of thep-electrode first layer.

The following materials are commonly used for the p-electrode firstlayer (not shown). For example, a metal and alloy of Ag, Zn, Ni, Pt, Pd,Rh, Ru, Os, Ir, Ti, Zr, Hf, V, Nb, Ta, Co, Fe, Mn, Mo, Cr, W, La, Cu andY, and a single film or stacked film of, for example, conductive oxidessuch as ITO, ZnO and SnO₂ may be used. With respect to the p-electrodesecond layer (not shown), for example, Pt, Au and Ni—Ti—Au basedelectrode material may be used.

Specifically not shown, if the p-electrode 30 consists of a two-layerstructure of p-electrode first layer/p-electrode second layer, a stackedlayer structure of, for example, Pt/Au, Pd/Au, Rh/Au and Ni/Au may beused. If the p-electrode 30 consists of a three-layer structure byinserting a p-electrode third layer between the p-electrode first layerand the p-electrode second layer, a stacked layer structure of, forexample, Ni/Pt/Au, Pd/Pt/Au and Rh/Pt/Au may be used. Furthermore, ifthe p-electrode 30 consists of a four-layer structure by inserting ap-electrode third layer and p-electrode fourth layer between thep-electrode first layer and the p-electrode second layer, a stackedlayer structure of, for example, Ag/Ni/Ti/Pt may be used.

(Passivation Film)

A passivation film 40 is made of a transparent material which has arefractive index lower than that of a p-type semiconductor layer 53 andformed on a plane identical to that of the p-electrode 30 at portionswhere the p-electrode 30 is not formed and. The passivation film 40 ismade of an insulator film, preferably made of an oxide film. Thepassivation film 40 is made of, for example, SiO₂ or ZrO₂.

The passivation film 40 may be formed by a well known method, such assputtering, ECR (Electron Cyclotron Resonance) sputtering, CVD (ChemicalVapor Deposition), ECR-CVD, ECR-plasma CVD, evaporation and EB (ElectronBeam). The passivation film 40 is preferably formed by, for example, ECRsputtering, ECR-CVD and ECR-plasma CVD.

(Semiconductor Stack)

A semiconductor stack 50 is made of, for example, GaN-based compoundsemiconductor (for example, GaN, AlGaN, InGaN and AlGaInN). Especially,GaN is preferable because an etched surface of GaN has a finecrystalline surface. The semiconductor stack 50 is formed bysequentially stacking an n-type semiconductor layer 51, a light emittinglayer 52 and a p-type semiconductor layer 53 in this order from the sideof a light extraction surface opposite to a surface to be mounted on thesubstrate 10. Meanwhile, the semiconductor stack 50 is generallyexpressed by In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1, 0≦x+y≦1).

On the light extraction surface, at least one concave 90 is formed. Inthe embodiment, the light extraction surface is a surface of the n-typesemiconductor layer 51. That is, the concave 90 is disposed on then-type semiconductor layer 51. The concave 90 will be described later indetail.

The n-type semiconductor layer 51 is made of GaN containing, forexample, Si, Ge and O (oxygen) as an n-type impurity. The n-typesemiconductor layer 51 may be formed by a plurality of layers.

The light emitting layer 52 is made of, for example, InGaN.

The p-type semiconductor layer 53 is made of, for example, GaNcontaining Mg as a p-type impurity.

On the light extraction surface of the semiconductor stack 50, at leastone electrode is formed. In the embodiment, the light extraction surfaceis formed on a surface of the n-type semiconductor layer 51. Therefore,the electrode formed on the extraction surface is the n-electrode 60.

(n-Electrode)

In the example shown in FIG. 1, the n-electrode 60 is formed on an areaother than the concave 90 on the light extraction surface. In theembodiment, since the light extraction surface is a surface of then-type semiconductor layer 51, the n-electrode 60 is electricallyconnected to the area other than the concave 90 on the upper surface ofthe n-type semiconductor layer 51. The n-electrode is connected to theoutside by a bonding wire. The n-electrode 60 consists of a metal stack,for example, Ti/Pt/Au, Ti/Pt/Au/Ni, Ti/Al, Ti/Al/Pt/Au, W/Pt/Au andV/Pt/Au, which are formed by stacking the metals in this order on theupper side of the n-type semiconductor layer 51. Meanwhile, then-electrode 60 may be composed of an ohmic contact and pad electrode.

(Passivation Film)

A passivation film 70 is formed by a material identical to that of theabove-mentioned passivation film 40. Namely, for example, if thepassivation film 40 is made of SiO₂, the passivation film 70 is alsomade of SiO₂, and compositions of the both films may be slightlydifferent each other depending on a forming method thereof. Thepassivation film 70 covers an upper surface of the n-electrode 60 excepta wire bonding area, an inside of the concave 90 including the innercircumferential surface, a surface of the n-type semiconductor layer 51and a side face of the semiconductor stack 50.

(Backside Metallization Layer)

A backside metallization layer 80 is formed on a surface of thesubstrate 10 opposite to a surface on which a metallization layer 20 isformed, and functions as an ohmic electrode. The backside metallizationlayer 80 is made of a metal stack, for example, TiSi₂/Pt/Au, which isformed by sequentially stacking the metals in this order from the upperside in FIG. 1.

(Concave Structure)

The concave 90 has, as shown in FIG. 2, two slopes 93, 94 each having adifferent slope angle in a direction that a diameter of the concave 90becomes narrower toward a bottom surface 92 from an opening 91. Theslope angle is formed to become gentler as a position of the slope iscloser to the opening 91 of the concave 90. Namely, the concave 90becomes narrower as a position leaves from a surface of the n-typesemiconductor layer 51, and a side surface of the concave 90 has the twoslopes 93, 94 having respective different narrowing angles θ₁, θ₂.Meanwhile, FIG. 2 corresponds to a cross section taken along X-X line ofFIG. 3.

In the two slopes, a slope (slope 93) having a gentler slope angle closeto the opening 91 is a surface having irregularities 95, and the slope94 having a steeper slope angle has a flat surface. In the example shownin FIG. 2, the irregularities 95 identical to that of the slope 93 arealso formed on a bottom of the concave 90. In addition, in the example,a plurality of the concaves 90 are disposed on the light extractionsurface, adjacent openings of the plurality of the concaves 90 areseparated from each other, and a separating area 201 is also providedwith the irregularities 95.

An average surface roughness (Ra) corresponding to a size of theirregularities 95 formed on the slope 93 is 10 to 100 nm, preferably 15to 60 nm, and most preferably 20 to 40 nm. Meanwhile, a depth of theconcave 90 is defined as described later, and it is, for example,several micrometers (μm). Therefore, the irregularities 95 formed on theslope 93 are called micro irregularities.

The average surface roughness (Ra) in the present invention may bemeasured using a scanning probe microscope (SPI3800N manufactured bySII).

In addition, the average surface roughness (Ra) can be measured from across section of a surface to be measured (hereinafter, referred to asmeasurement surface) based on a definition of arithmetical meanroughness Ra of JIS B0601.

According to the definition of JIS B0601, an average line (wave line) isacquired from a profiling curve of the measurement surface, and theaverage line is subtracted from the profiling curve. That is, byreplacing the average line with a straight line, a roughness curve isacquired. In addition, according to a coordination system defined by JISB0601, a direction identical to the average line which is replaced witha straight line is set to an X-axis and a direction orthogonal to theX-axis and parallel to the cross section is set to a Z-axis.

By sampling a standard length l from the roughness curve in the X-axisdirection, an average line of the sampling portion can be expressed bythe following formula (1).

$\begin{matrix}{Z_{0} = {\frac{1}{l}{\int_{0}^{l}{{Z(x)}\ {\mathbb{d}x}}}}} & {{formula}\mspace{14mu}(1)}\end{matrix}$

In this case, the average surface roughness (Ra) is a value which isacquired by averaging absolute values of deviations (differences)between Z(x) and Z₀, and can be acquired by the following formula (2).

$\begin{matrix}{{Ra} = {\frac{1}{l}{\int_{0}^{l}{{{{Z(x)} - Z_{0}}}\ {\mathbb{d}x}}}}} & {{formula}\mspace{14mu}(2)}\end{matrix}$

Specifically, a measurement surface is observed at a cross sectiondescribed above using a microscope which is capable of highmagnification such as, for example, TEM, and an average line and aroughness curve are acquired from the profiling curve. An arbitrary 500nm span is selected on the X-axis, in the selected span, a hundredX-values (X₁ to X₁₀₀) are set at constant intervals, and a Z-value(Z(X₁) to Z(X₁₀₀)) at each X-value is measured. Z₀ can be acquired fromthe measured Z-value using formula (3).Z ₀=(1/100)×{Z(x ₁)+Z(x ₂)+Z(x ₃)+ . . . +Z(x ₁₀₀)}  formula (3)

An average surface roughness (Ra) can be acquired using the acquired Z₀and the following formula (4).Ra=(1/100)×{|Z(x ₁)−Z ₀ |+|Z(x ₂)−Z ₀ |+ . . . +|Z(x ₁₀₀)−Z ₀|}  formula(4)

In the embodiment, the narrowing angles θ₁, θ₂ are defined as an anglewhich is formed by the horizontal plane of the semiconductor stack 50and the respective slopes 93, 94. The narrowing angle θ₁ on the surfaceside is larger than 0° and less than 60°, preferably 20 to 50°. Thenarrowing angle θ₂ on the side of the light emitting layer 52 is largerthan 60° and less than 90°, preferably 60 to 80°. Here, if a differencebetween the narrowing angles θ₁, θ₂ is considerably large, a lightemitted from the light emitting layer 52 at various angles can be easilyextracted by efficiently reflecting and transmitting the light by theslopes 93, 94. For example, as shown in FIG. 2, if the two slopes 93, 94are clearly distinguished, the narrowing angles θ₁, θ₂ and θ₂−θ₁ becomeθ₁=45°, θ₂=70° and θ₂−θ₁=25°. If the narrowing angles θ₁, θ₂ are set inthe foregoing ranges, it is relatively easy to set the difference θ₂−θ₁between the slope angles to a considerably large value.

If the n-type semiconductor layer 51 is formed by a plurality of layers,each of the slopes 93, 94 is preferably made of GaN. Further, it is morepreferable if a concentration of n-type impurity (for example, Si) dopedinto the GaN of each of the slopes 93, 94 is varied. In addition, it maybe possible that the slope 93 on the surface side is made of undoped GaNand the slope 94 on the side of the light emitting layer 52 is made ofsi-doped GaN.

A depth D of the concave 90 has a length not less than half of athickness H that is a distance from the light extraction surface to thelight emitting layer 52 and less than the thickness H(H/2≦D<H). Inaddition, in the depth D of the concave 90, it is preferable that adepth D1 corresponding to the slope 93 is deeper than a depth D2corresponding to the slope 94 for easy forming of the concave 90.

A bottom size W (diameter W) of the concave 90 is not less than ⅕ andnot more than ½ of an opening size L (maximum width L) of the opening 91(L/5≦W≦L/2), while depending on the narrowing angles θ₁, θ₂, andpreferably not less than ⅓ and not more than ½ (L/3≦W≦L/2). Here, if adifference between the bottom size W of the concave 90 and the openingsize L of the opening 91 is considerably large, a light emitted from thelight emitting layer 52 at various angles can be easily extracted byefficiently reflecting and transmitting the light by the slopes 93, 94.For example, if the bottom size W of the concave 90 is set to ⅓ of theopening size L of the opening 91, since the depth D1 corresponding tothe slope 93 in the depth D of the concave 90 can be set deeper than thedepth D2 corresponding to the slope 94, while setting the narrowingangles θ₁, θ₂ in the foregoing ranges, a formation of the concave 90becomes easy. Because of the above-mentioned reason, the narrowingangles θ₁, θ₂ are set in the foregoing ranges.

As shown in the plane view of FIG. 3, with respect to an arrangement ofthe openings 91 of the concaves 90, respective centers of a plurality ofconcaves 90 are in line in each row and in line every other column.Symbols 90 a, 90 b, 90 c, 90 d, 90 e, 90 f and 90 g are given to theseven concaves shown in FIG. 3 for convenience. In FIG. 3, the concave90 a is surrounded by six concaves 90 b to 90 g at predeterminedintervals in the clockwise direction. As shown in FIG. 3, each openingof the concaves 90 a to 90 g is surrounded by an area 201 of the lightextraction surface. Therefore, a current diffusion on the lightextraction surface is good. It is noted that the area 201 may be a flatsurface or a curved surface.

As described above, since a size of the micro irregularities 95 and asize of the concave 90 are extremely different, the micro irregularities95 are shown by fine dots in the plane view of FIG. 3 and in FIG. 4.Then, only an area of the steep slope 94 in the concave 90 is shown withwhite (without fine dots). This is the same with any other drawings suchas FIG. 13. It is noted that the micro irregularities 95 are overdrawnfor convenience for explanation as shown in FIG. 2.

[Characteristics of Semiconductor Light Emitting Device]

(Light Extraction Efficiency)

FIG. 5A to FIG. 5D are cross sectional views schematically showing acomparison between a concave of a semiconductor light emitting deviceaccording to the embodiment and concaves other than the embodiment. FIG.5A shows an example (example of embodiment) of a cross section of theconcave 90 in the semiconductor light emitting device 1 according to theembodiment, and FIG. 5B to FIG. 5D show cross sections of concaves eachhaving a different structure. A COMPARATIVE EXAMPLE 1 shown in FIG. 5Bis a shallow concave 190A with a slope having only a gentle slope angle.In the shallow concave 190A, if a light emitted from a light emittinglayer in a semiconductor layer obliquely enters the slope, the lighttransmits in an oblique direction depending on a difference of arefractive index between an inside and outside substances of the slope,and as a result, the light does not propagate in the right upperdirection. Namely, both the light extraction efficiency and lightdistribution become poor.

A COMPARATIVE EXAMPLE 2 shown in FIG. 5C is a deep concave 190B with aslope having only a steep slope angle. If a light emitted from a lightemitting layer in a semiconductor layer enters the slope from inside thedeep concave 190B, the light is extracted outside after repeating areflection at the slope. However, if an electrode is disposed on thelight extraction surface of the deep concave 190B, a current diffusionfrom the electrode becomes poor.

On the other hand, the concave 90 that is an example of the embodimentshown in FIG. 5A has two slopes 93, 94 each having a different slopeangle. The slope 93 having a gentle slope angle is a surface which isprovided with the irregularities 95 and the slope 94 having a steepslope angle is a flat surface. Therefore, in the concave 90, if a lightemitted from a light emitting layer in a semiconductor layer obliquelyenters the slope 93 having a gentle slope angle and micro irregularities95, the light is scattered by the micro irregularities 95, and as aresult, a component of the light propagating in the right upperdirection increases. In addition, if a light emitted from a lightemitting layer in a semiconductor layer enters the slope 94 having asteep slope angle from inside the concave 90, the light is extractedoutside after repeating reflection at the slope 94. Namely, both thelight extraction efficiency and light distribution become good. Inaddition, when an electrode is disposed on the light extraction surfaceof the concave 90, since the slope 93 having a gentle slope angle existsclose to the electrode, a current flowing in the right downwarddirection from the electrode can be reduced in comparison with the deepconcave 190B (COMPARATIVE EXAMPLE 2) where a slope having only a steepslope angle is disposed. As a result, a current diffusion from theelectrode can be prevented from degrading. Furthermore, in theembodiment, since the micro irregularities 95 are also disposed on thebottom surface 92 and the area 201, when a light emitted from a lightemitting layer in a semiconductor layer enters the bottom surface 92 andthe area 201, the light is scattered by the micro irregularities 95.Therefore, the light extraction efficiency and light distribution becomegood in comparison with a flat surface without the micro irregularities95.

On the other hand, a COMPARATIVE EXAMPLE 3 shown in FIG. 5D is afictitious concave 190C, where in the two slopes each having a differentslope angle, a slope 193 having a gentle slope angle is a flat surfaceand a slope 194 having a steep slope angle is a surface provided withthe irregularities 95. In this case, in the fictitious concave 190C, ifalight emitted from a light emitting layer in a semiconductor layerobliquely enters the slope 193 having a flat surface and gentle slopeangle, the light is not scattered, and as a result, a component of thelight propagating in the right upper direction is not increased. Inaddition, if a light emitted from a light emitting layer in asemiconductor layer enters the slope 194 provided with theirregularities 95 and having a steep slope angle, the light isscattered. Therefore, a component of the light propagating downwardincreases and a reflection at the slope 194 becomes poor. As a result,the light is likely to be less extracted outside. Namely, both the lightextraction efficiency and the light distribution become poor.

(Light Distribution)

FIG. 6 is a graph showing a relationship between a roughness of a lightextraction surface and a light directionality. FIG. 6 shows a lightdirectionality with a polar coordinate, and a radial direction indicatesa light intensity and a circumferential direction shows an angle. Here,with respect to the circumferential direction, a negative direction ofthe horizontal axis (X-axis) indicates 0° (zero degree) directivityangle and a positive direction of the horizontal axis (X-axis) indicates180° directivity angle with reference to 90° (vertical axis: Y-axis)directivity angle. In addition, 0 to 180° directivity angles indicateangles measured in the longitudinal direction (φ=90°) of the n-electrode60 in plane view. In FIG. 6, a dotted line indicates a case where thelight extraction surface is flat and a solid line indicates a case wherethe light extraction surface is provided with concaves/convexes.However, an absolute value at 90° directivity angle is normalized to “1”for making clear the difference between the both light distributions.

As shown in FIG. 6, if the light extraction surface is provided with theconcaves/convexes (solid line in FIG. 6), the light intensity is largestat 90° directivity angle. On the other hand, if the light extractionsurface is flat (dotted line in FIG. 6), the light intensity is largestat 50° and 130° directivity angles, which is 1.8 times larger than thelight intensity at 90° directivity angle. Namely, if the lightextraction surface is provided with concaves/convexes, the lightdistribution is improved in comparison with a flat surface.

FIG. 7 is a graph showing an example of a light directionality of asemiconductor light emitting device according to the embodiment. FIG. 7is drawn with the same scheme as FIG. 6. In FIG. 7, a solid lineindicates data of the semiconductor light emitting device 1 according tothe embodiment, that is, the data in a case where the semiconductorlight emitting device 1 is provided with the micro irregularities 95like the concave 90. In addition, a dotted line indicates data in a casewhere the semiconductor light emitting device 1 is not provided with themicro irregularities 95. As shown in FIG. 7, it is known that anexistence of micro irregularities effects on a light intensity at around90° directivity angle. Namely, if the micro irregularities 95 aredisposed on a concave, a light intensity increases at around 90°directivity angle, and the light distribution is improved.

It is noted that in FIG. 7, absolute values at 60° and 120° directivityangles are normalized to “1” for making clear the difference between thetwo data. In addition, 0° to 180° directivity angles are measured in thelateral direction of n-electrode 60 (φ=0°) in plane view. A lightintensity in the radial direction per unit length in FIG. 7 is half ofFIG. 6.

[Fabrication Process of Semiconductor Light Emitting Device]

A fabrication method of the semiconductor light emitting device shown inFIG. 1 will be explained by referring to FIG. 8A to FIG. 8F and FIG. 9Ato FIG. 9E (see FIG. 1 to FIG. 4 as appropriate). FIG. 8A to FIG. 8F andFIG. 9A to FIG. 9E are cross sectional views schematically showing afabrication process of the semiconductor light emitting device shown inFIG. 1.

First, as shown in FIG. 8A, a n-type semiconductor layer 51, a lightemitting layer 52 and a p-type semiconductor layer 53 (see FIG. 1) aregrown in this order on a substrate 100 for growing semiconductor layers(hereinafter, referred to as semiconductor growing substrate 100) toform a semiconductor stack 50. The semiconductor growing substrate 100is a substrate which is removed in a later process and made of asapphire having one of a C-plane, R-plane and A-plane as a main plane.Meanwhile, a substrate different from sapphire may be used as thesemiconductor growing substrate 100. For example, an insulator substratesuch as spinel (MgAl₂O₄), SiC (including 6H, 4H and 3C), ZnS, ZnO, GaAsand an oxide substrate whose lattice matches with those of nitridesemiconductors may be used as the semiconductor growing substrate 100,which are materials on which nitride semiconductors can be grown andwell known as a substrate material.

Next, as shown in FIG. 8B, a p-electrode first layer and a p-electrodesecond layer, which are not shown, are grown in this order on an uppersurface of the semiconductor stack 50 (surface of p-type semiconductorlayer 53) to form a p-electrode 30 using magnetron sputtering. Next, asshown in FIG. 8C, a passivation film 40 is formed between thep-electrodes 30, 30 using ECR sputtering. Next, as shown in FIG. 8D, ametallization layer 21 on an epitaxial layer side is grown on thep-electrode and the passivation film 40. In addition, before or after,or in parallel with forming the metallization layer 21 on the epitaxiallayer side, a metallization layer 22 on a substrate side is grown on thesubstrate 10 as shown in FIG. 8E. Next, as shown in FIG. 8F, themetallization layer 22 on the substrate side and the metallization layer21 on the epitaxial side are bonded by turning back the substrate 10 onwhich the metallization layer 22 on the substrate side is grown. Ametallization layer 20 is formed by the bonded metallization layers 21and 22 on the epitaxial layer side and on the substrate side,respectively.

Next, as shown in FIG. 9A, the semiconductor growing substrate 100 isremoved from the semiconductor stack 50. As shown in FIG. 9B, an uppersurface (surface of n-type semiconductor layer 51) of the semiconductorstack 50, which is an uppermost surface due to turning back of thesubstrate 10 from which the semiconductor growing substrate 100 isremoved, is polished by CMP (Chemical Mechanical Polishing). Next, asshown in FIG. 9C, the concave 90 is formed on the upper surface (surfaceof n-type semiconductor layer 51) of the semiconductor stack 50 by aprocess described later.

After forming the concave 90, as shown in FIG. 9D, the n-electrode 60 isformed on an area where the concave 90 is not formed on the uppersurface (surface of n-type semiconductor layer 51) of the semiconductorstack 50.

Next, as shown in FIG. 9E, an inside of the concave 90 including acircumferential surface and the upper surface (surface of n-typesemiconductor layer 51) of the semiconductor stack 50 are covered with apassivation film 70. It is noted that as shown in FIG. 1, a side face ofthe semiconductor stack 50 is also covered with the passivation film 70.Next, a backside metallization layer 80 as an ohmic electrode is formedon the surface of the substrate 10, which became an uppermost surface(bottommost surface in FIG. 1) by turning back the substrate 10 (notshown), then, the wafer is diced. Namely, the wafer is divided intobars, a mirror is formed on an end face of a resonator, the bars arecleaved into chips and wires are bonded to the n-electrode 60 and asurface of the backside metallization layer 80 to fabricate thesemiconductor light emitting device 1 shown in FIG. 1.

[Forming Process of Concave]

Next, a forming process of the concave 90 will be explained by referringto FIG. 10 (see FIG. 2 as appropriate). FIG. 10 is a flowchart showing aforming process of a concave shown in FIG. 9C.

A resist pattern having an opened area corresponding to the concave 90is formed on an upper surface (surface of n-type semiconductor layer 51)of the semiconductor stack 50 by, for example, photolithography (stepS1). After patterning the resist, a post-bake (heat treatment) isconducted at, for example, 180° C. so that an end portion of the resistpattern is deformed to incline by the heat treatment of the resistpattern, as well as abridge formation of the resist is promoted (stepS2). Through the post-bake, a structure where the end portion of theresist pattern is collapsed to incline from the periphery toward thearea to become the concave 90 is formed. Namely, an inclined portionformed by the collapse of the pattern end portion is formed just insidefrom a periphery of the etching area. As a result, an area having a lowetching rate in comparison with no resist area is formed.

Subsequently, an area to become the concave 90 is dry-etched by, forexample, RIE (Reactive Ion Etching) using the resist mask (step S3).Since the area having a low etching rate is formed on the upper surface(surface of n-type semiconductor layer 51) of the semiconductor stack50, the surface is etched so that there are two inclined stages.

Next, the resist pattern used for the etching as a mask is stripped(step S4). A well known common method is used for stripping the resistpattern. For example, a photoresist stripping liquid may be used, or anapparatus which irradiates the photoresist with, for example, oxygenplasma (asking) such as an asher may be used.

Next, an entire wafer surface is wet-etched without patterning.Regarding an etchant of wet-etching, as an anisotropic etchant, forexample, KOH solution, TMAH (tetramethyl ammonium hydroxide) alkalinesolution and EPD (ethylene diamine pyrocatechol) may be used. With theetching described above, the micro irregularities 95 are formed on anarea other than the slope 94 having a steep slope angle. Namely, themicro irregularities 95 are formed on the slope 93 having a gentle slopeangle, the bottom surface 92 and the area 201 on the light extractionsurface. It is noted that the micro irregularities 95 are formed beneaththe n-electrode 60 (see FIG. 1 and FIG. 9E)

According to the semiconductor light emitting device 1 of theembodiment, since the concave 90 is disposed on the light extractionsurface that is the upper surface of the semiconductor stack 50, itbecomes easy to efficiently extract a light outside from a surface onthe light extraction side in comparison with a case where a convex isdisposed instead of the concave 90. In addition, in the concave 90,since the slope 93 having a gentle slope angle is provided with themicro irregularities 95 and the slope 94 having a steep slope angle hasa flat surface, the light directionality becomes good. Furthermore, acurrent diffusion from the n-electrode 60 becomes good due to existenceof the concave 90. In addition, in the semiconductor light emittingdevice 1, since the two slopes 93, 94 each having a different slopeangle in the direction that a diameter of the concave 90 becomesnarrower toward the bottom surface 92 from the opening 91 are formed tohave a gentler angle as the slopes 93, 94 become closer to the opening91 of the concave 90, a light emitted from the light emitting layer 92is easily extracted outside by reflection. In addition, in thesemiconductor light emitting device 1, since the concave 90 has thebottom surface 92, a re-entering of a light, which is emitted from thesemiconductor stack 50 (semiconductor layer), into the semiconductorstack 50 can be reduced.

The embodiment of the present invention has been explained. However, thepresent invention is not limited to the embodiment described above andcan be embodied in various forms without departing from the spirits ofthe invention. For example, the light extraction surface of thesemiconductor stack 50 was the n-type semiconductor layer 51. However,the light extraction surface may be the p-type semiconductor layer 53and the concave 90 may be disposed on the p-type semiconductor layer 53.Meanwhile, it is preferable that the concave 90 is disposed like theembodiment since the concave 90 can be formed deeper. In addition, inthe embodiment, the two slopes 93, 94 were disposed in the concave 90.However, a concave which has not less than three slopes has the sameadvantages as long as a narrowing angle (slope angle) of a slope isformed to be gentler as the slope becomes closer to the opening 91 ofthe concave 90. In this case, a narrowing angle corresponding to atleast one slope to be disposed between the slopes 93, 94 is set betweenθ₁ and θ₂ as appropriate so that a difference among the narrowing anglesbecomes as equal as possible, while the narrowing angle θ₁ correspondingto the slope on the surface side and the narrowing angle θ₂corresponding to the slope on the light emitting layer 52 are set in theabove-mentioned ranges. As a result, a light emitted from the lightemitting layer 52 at various angles can be easily extracted byefficiently reflecting and transmitting by not less than three slopes.In addition, in the embodiment, the n-electrode 60 is disposed in anarea without the concave 90. However, the n-electrode 60 may be disposedon the concave 90. It is noted that the n-electrode 60 is preferablydisposed like the embodiment because a current in the semiconductorstack 50 (semiconductor layer) is easily diffused. A material composingthe semiconductor stack 50 of the semiconductor light emitting device 1is not limited to nitride semiconductors.

In the embodiment, the bottom surface 92 of the concave 90 was flat.However, it is not always necessary that the bottom surface is flat, butthe bottom surface, for example, may be a curved surface protrudingdownward. In addition, in the present invention, the bottom surface 92is not always necessary on a bottom of the concave 90, but the slope 94may be narrowed in a reverse circular cone toward the bottom.

In addition, in the embodiment, a slope angle of each of the slopes 93,94 of the concave 90 is formed to be gentler as the slope is closer tothe opening 91 of the concave 90. However, the present invention is notlimited to this. Like a concave 90B shown in FIG. 12, a slope 93B havinga steep slope angle and a slope 94B having a gentle slope angle may beformed in this order from the opening. In this case, a slope having agentle slope angle is provided with the irregularities 95 and a slopehaving a steep slope angle has a flat surface as the foregoingembodiment. Therefore, as shown in FIG. 12, the micro irregularities 95are continuously formed from the slope 94B to the bottom surface 92. Asemiconductor light emitting device having such a structure as theconcave 90B also has good light extraction efficiency and good lightdistribution.

In addition, with respect to an opening shape of a concave in planeview, various modifications may be possible. The variations of the shapewill be explained below.

First Modified Example

FIG. 13 is a view schematically showing a plane, X-X cross section andY-Y cross section of a first modified example of an opening of aconcave, and FIG. 14 is a perspective view schematically showing apartial cross section of the first modified example taken along Y-Ycross section shown in FIG. 13. In the first modified example, aconfiguration of the opening is the same as that of concave 90 shown inFIG. 3 except that a shape of a plurality of concave openings is ahexagon and that a density of the concave opening is not so high asshown in FIG. 3. Namely, for example, in the concave 90 a, each vertexof the regular hexagon as an opening is in contact with six concaves 90b to 90 g surrounding the concave 90 a. In other words, the concave 90 ais surrounded by six regular triangles shown in plane view of FIG. 13.These six triangles are located on the light extraction surface.Therefore, a current diffusion on the light extraction surface becomesgood. It is noted that each vertex of the regular hexagon as an openingmay be separated by a predetermined distance from the six surroundingconcaves.

Second Modified Example

FIG. 15 is a view schematically showing a plane, X-X cross section andY-Y cross section of a second modified example of an opening of aconcave, and FIG. 16 is a perspective view schematically showing apartial cross section of the second modified example taken along Y-Ycross section shown in FIG. 15. In the second modified example, anarrangement of openings of a plurality of concaves is shown in FIG. 15,which is a close-packed structure. Namely, for example, the concave 90 ais in contact with six surrounding concaves by respective sides of aregular hexagon as an opening. In addition, the concave 90 a is incontact with six surrounding concaves at respective vertexes of theregular hexagon as an opening. Here, as shown in FIG. 16 which is a Y-Yarrow cross section of FIG. 15, for example, each side of the regularhexagon as an opening of the concave 90 a is straight and on the lightextraction surface. As described above, in the second modified example,each side and each vertex of the opening of the concave are in contactwith adjacent concaves by forming a close-packed structure. In thesemiconductor light emitting device having the concave described above,many concaves can be efficiently arranged and integrated in a smallsurface.

Third Modified Example

FIG. 17 is a view schematically showing a plane, X-X cross section andY-Y cross section of a third modified example of an opening of aconcave, and FIG. 18 is a perspective view schematically showing apartial cross section of the third modified example taken along Y-Ycross section of FIG. 17. In the third embodiment, each side of aregular hexagon as an opening of the concave 90 a is not a straightline, but a v-shaped line having a vertex (bottom) at the center of theside. Namely, only respective vertexes of the regular hexagon in planeview are located on a plane identical to the light extraction surface.Here, an upper slope (slope 93, see FIG. 2) is located below a valley ofthe V-shape. In other words, the V-shape is within the upper slope(slope 93, see FIG. 2). Therefore, there is an advantage that a currentcan be easily and uniformly diffused in comparison with a case where thev-shape is not formed. Other characteristics are the same with those ofthe second modified example and the explanation will be omitted.

Other Modified Examples

A circle and hexagon in plane view as a shape of the opening 91 of theconcave 90 have been exemplified. Of course, a polygon and ellipsoidsuch as a triangle and quadrangle can be surely used. A circle orhexagon which is capable of forming a close-packed structure ispreferable for efficiently diffusing a current and improving the lightextraction efficiency.

Embodiment

A semiconductor light emitting device according to the embodiment of thepresent invention was fabricated for confirming advantages of thepresent invention. Specifically, a semiconductor light emitting device 1was fabricated according to the fabrication processes shown in FIG. 8Ato FIG. 10.

For fabricating the semiconductor light emitting device 1, a sapphiresubstrate was used for the substrate 100 for growing semiconductorlayers (semiconductor growing substrate 100). In addition, as asubstrate 10, a silicon (Si) wafer substrate 400 μm thick was used. Thefollowing layers were grown on the sapphire substrate for forming thesemiconductor stack 50. First, a n-type cladding layer composed ofSi-doped AlGaN and a n-type light guiding layer composed of GaN weregrown on the sapphire substrate. With the processes described above, then-type semiconductor layer 51, 4000 nm thick was formed. Subsequently, abarrier layer composed of Si-doped In_(0.05)Ga_(0.95)N and a well layercomposed of undoped In_(0.1)Ga_(0.9)N were alternately grown two times,and further another barrier layer was grown thereon to form amultiple-quantum well (MQW) structure that is the light emitting layer52. Next, a p-type electron confinement layer composed of Mg-dopedAlGaN, a p-type light guiding layer composed of undoped GaN, a p-typecladding layer composed of a super lattice layer which was formed byalternately growing a undoped Al_(0.16)Ga_(0.84)N layer and a Mg-dopedGaN layer and a p-type contact layer composed of Mg-doped p-type GaNwere grown to form the p-type semiconductor layer 53. After that, thewafer was annealed at 700° C. in nitrogen atmosphere for furtherlowering a resistance of the p-type semiconductor layer 53.

The p-electrode 30, 400 nm thick has a layer structure of Ag/Ni/Ti/Pt inthis order from the semiconductor stack 50. The passivation film 40, 400nm thick was made of SiO₂. The metallization layer 21 on the epitaxiallayer side has a thickness of 1400 nm, and Ti/Pt/Au/Sn/Au were grown inthis order from the bottom in FIG. 8D. The metallization layer 22 on thesubstrate side has a thickness of 653 nm, and Au/Pt/TiSi₂ or TiSi₂/Pt/Pdwere grown in this order. A depth D of the concave 90 formed on then-type semiconductor layer 51 using, for example, photolithography anddry-etching was 2500 nm. The micro irregularities 95 having a surfaceroughness of 30 nm were formed by immersing the wafer into TMAH. Then-electrode 60, 1300 nm thick has a layer structure of Ti/Pt/Au in thisorder from the upper side of the n-type semiconductor layer 51. Thepassivation layer 70, 400 nm thick was made of SiO₂. The backsidemetallization layer 80 has a thickness of 753 nm, and TiSi₂/Pt/Au weregrown in this order on the substrate 10. The semiconductor lightemitting device 1 configured as described above shows the lightdistribution indicated with solid line in FIG. 7.

INDUSTRIAL APPLICABILITY

A semiconductor light emitting device according to the present inventioncan be utilized in various fields, for example, lighting, exposure,display, various kinds of analysis and optical network.

What is claimed is:
 1. A semiconductor light emitting device comprising:a semiconductor stack comprising an n-type semiconductor layer, a p-typesemiconductor layer, and a light emitting layer between the n-typesemiconductor layer and the p-type semiconductor layer, wherein thesemiconductor stack includes at least one concave structure, wherein theconcave structure has two slopes each having a different slope angle ina direction that a diameter of the concave structure becomes narrowertoward a bottom of the concave structure from an opening of the concavestructure, wherein an upper surface of the semiconductor stack includesa higher planar portion outside the concave structure and a lowersurface of the semiconductor stack includes a lower planar portioninside the concave structure, wherein the two slopes connect the higherplanar portion to the lower planar portion, wherein a slope, of the twoslopes, having a gentler slope angle is provided with irregularities anda slope, of the two slopes, having a steeper slope angle is a flatsurface, and wherein a depth of the concave structure has a length notless than half of a thickness between the upper surface and a topsurface of the light emitting layer and less than the thickness betweenthe upper surface and the top surface of the light emitting layer. 2.The semiconductor light emitting device according to claim 1, whereinthe slope of the concave structure having the gentler slope angle iscloser to the opening of the concave structure than the slope having thesteeper slope angle.
 3. The semiconductor light emitting deviceaccording to claim 1, wherein the lower surface is provided withirregularities.
 4. The semiconductor light emitting device according toclaim 1, wherein a plurality of the at least one concave structure isdisposed on a light extraction surface of the semiconductor stack,wherein adjacent openings of the plurality of the concave structures areseparated from each other by the upper surface, and wherein the uppersurface is provided with irregularities.
 5. The semiconductor lightemitting device according to claim 1, wherein an electrode is disposedon a light extraction surface of the semiconductor stack.
 6. Thesemiconductor light emitting device according to claim 1, wherein ashape of the opening of the concave structure is a circle in plan viewof the semiconductor stack.
 7. The semiconductor light emitting deviceaccording to claim 1, wherein a p-electrode is disposed on a surface ofthe semiconductor stack opposite to a surface where the concavestructure is disposed.
 8. The semiconductor light emitting deviceaccording to claim 1, wherein the n-type semiconductor layer includesthe concave structure.
 9. The semiconductor light emitting deviceaccording to claim 1, wherein the light emitting device furthercomprising a passivation film comprising an insulating material.